A critical gap exists in how massive weight loss impacts postural stability and fall risks in obese adults with knee osteoarthritis (OA). Persistenc of this gap delays reducing fall risks and improving motor function. The long-term goals are to create innovative interventions that minimize fall risks and to develop new methods of detecting fall risks in this population. The knowledge gained will be used to conduct a randomized controlled trial that assesses speed of recovery in postural stability following bariatric surgery and to develop obstacle and stepping tasks to be used as diagnostic tools for identifying fall risks. The objective of the proposed study is to test if massive weight loss after bariatric surger in adults with knee OA is sufficient to improve key factors contributing to postural instability an fall risks: knee pain, atypical gait, and a decreased ability to control the center of mass (COM). Despite the known effects of obesity on postural stability, we know little about if adults who have massive weight loss improve postural stability based on spatial-temporal gait and COM measures. There is also limited evidence on if improvements in postural stability occur on flat ground and tasks beyond flat ground walking. Most importantly, there are few objective measures after bariatric surgery related to activities outside of the lab. The hypothesis is that bariatric surgery patients will demonstrate better postural stability in obstacle crossing and in stepping up and down a stair than non-bariatric surgery patients, but will still show residual deficits in postural stability compared to normal weight adults. This hypothesis was formulated based on preliminary data collected in the principal investigator's and co-investigators' labs. There are two specific aims: 1) to examine if gait and COM measures associated with postural instability improve after bariatric surgery and 2) to determine if postural transition measures related to fall risks improve after bariatric surgery. With the first aim, an established paradigm will be used to test kinematics during flat ground walking, obstacle crossing, and stepping up and down a stair in the lab. Under the second aim, a proven method will be used to measure postural transitions during activities of daily living outside of the lab. The approach is innovatie because of its use of direct and objective measures of changes in gait, postural stability, and postural transitions during tasks inside and outside of the lab setting. The proposed research is significant because it can lead to a new science focused on quantification, detection, and intervention to improve postural stability and reduce fall risks in adults with knee OA who are obese.